The transmission capacity of fiber-optic communication systems has increased significantly by use of the wavelength division multiplexing (WDM) technique. In a WDM system, multiple channels, where each channel is differentiated by using a different wavelength of light, each carry modulated optical signals in a single optical fiber between transmitter and receiver nodes. In a typical optical communication system, it is desirable to have a few access nodes along the fiber path between the transmitter and receiver end terminals that have the ability to add and/or drop one or more optical channels. A node having this capability is often referred to as an optical add/drop multiplexer (OADM).
FIG. 1 illustrates a conventional OADM 110 arranged to drop and add only a single optical channel. OADM 110 has two input ports 120 and 130, and two output ports 140 and 150. Input port 120 carries multiplexed optical channels λ1 through λN from the communication line and input port 130 carries a local optical channel λi-add that is to be added to the fiber link. Output port 140 contains all the optical channels λ1 through λN from the input port 120, except the optical channel λi-drop that has been extracted and essentially replaced by λi-add. The dropped optical channel λi-drop emerges from output port 150.
Some simple OADM's of the type shown in FIG. 1 are fixed, in that only a preassigned optical channel can be added/dropped; in more sophisticated arrangements, a reconfigurable system architecture may be used to implement a tunable optical channel OADM that is able to change the wavelength that is added and/or dropped.
A different architecture is conventionally required when an access node in an optical communication system has to add/drop more than one channel. FIG. 2 illustrates a solution based on a cascade of single channel OADMs at the access node. The multiplexed optical channels are introduced at input port 220 of a first OADM 210-1. The output port 240 of OADM 210-1 is connected to the input port of a second OADM 210-2. The output port 260 of OADM 210-2 carries all the multiplexed optical channels to be transmitted on the communication channel. OADMs 210-1 and 210-2 have channel add ports 230-1 and 230-2 and channel drop ports 250-1 and 250-2, respectively. Each OADM may be of the fixed channel type or tunable channel type.
While FIG. 2 shows, for illustrative purposes, a solution with two OADMs that can add/drop one channel each, for a total of two channels, more than two OADMs can be inserted at the access node using the serial cascade approach. The cascading solution, however, suffers from a high through loss for the channels that have to pass all the OADMs in the cascade from the communication system input 220 to the output 260.
FIG. 3 illustrates another conventional solution based on an OADM 310 that can add and drop multiple channels within a single device. An input port 320 carries the multiplexed optical channels from the communication line while input port 330 carries the multiplexed local optical channels that are to be added to the fiber link. The local channels to be added, which are available from transmitters 380-1 through 380-N, are combined in a multiplexer 360 and applied to input 330. Output port 340 carries multiplexed optical channels consisting of all the added optical channels from input port 330 and the through channels from the system input port 320. The dropped optical channels emerge from output port 350 as a group of channels, and must be separated in a demultiplexer 370 before being available to receivers 390-1 through 390-N.
The multiple channel OADM of FIG. 3 eliminates the high through loss associated with the cascading solution of FIG. 2; however, it requires additional hardware for multiplexing (with multiplexer 360) and demultiplexing (with demultiplexer 370) the added and dropped channels. If the added and dropped channels are a fixed subset, then only the required subset of optical channel transmitters in transmitters 380-1 through 380-N and subset of optical receivers in receivers 390 through 390-N are populated. This is an efficient solution. However, in a dynamic optical communication system, the added and dropped channels can change over time, according to demand. Complete network flexibility necessitates full population of all the optical channel transmitters 380-1 through 380-N and receivers 390-1 through 390-N. This is a very expensive solution, as only a subset of channels will typically be used at any given time, while the others remain idle. Tunable transmitters and receivers cannot be used with the multiplexers and demultiplexers, due to the fixed channel assignment between the input and output ports of such devices. Passive combining and splitting can be used, but the power budget for that solution is impracticable.